Esters of saturated and unsaturated carboxylic acids can be produced by ester exchange, or transesterification, reactions. Transesterification reactions normally are carried out in the presence of a catalyst to accelerate the reaction desired. In the past, typical catalysts included materials such as sulfuric acid, toluenesulfonic acid, alkali metal alkoxides, or metal alkoxides such as those of titanium or aluminum. However, these catalysts suffer from a variety of drawbacks, especially with the esters of unsaturated carboxylic acids. For example, if strong mineral acids such as sulfuric acid or methanesulfonic acid are used, the reaction rates are generally quite slow and the formation of the transester product is normally accompanied by the formation of high concentrations of side-products. These by-products not only include Michael-addition products (addition of alcohol to C.dbd.C double bond) but also substantial amounts of polymeric products. In addition, primary or secondary alcohols may be dehydrated by strong acids, thus contaminating the product monomer with olefins derived from the starting alcohols.
On the other hand, alkali metal alkoxide catalysts (for example, sodium methoxide or potassium tert-butoxide), not only promote undesireable side reactions, but are also deactivated by the presence of water in the reaction solution. Therefore, it is necessary to continuously add catalyst to the reaction mixture. Furthermore, the catalyst must then be removed to avoid alkoxide-promoted polymerization or degradation during distillation or other thermal treatment of the products, especially if the products are unsaturated esters such as acrylic esters. Aluminum and titanium alkoxides also suffer from many of these same drawbacks. Titanate catalysts are especially sensitive to water (generally losing activity in mixtures containing greater than 500 ppm water), thus necessitating the same need to add more catalyst to the reaction. In addition, applications that require the catalyst to remain in solution (for example, when a monomeric product is not distilled) are hampered by the subsequent precipitation of hydroxides and oxides from the resultant product upon exposure to traces of water. Because of these problems with conventional catalysts, a need exists for an improved transesterification catalyst of high activity and selectivity and reduced sensitivity to water.
Some steps toward meeting this need have been taken in the art. For example, the utilization of titanium (Ti) and zirconium (Zr) transition metal complexes to catalyze transesterification reactions has been reported. Thus, as described in U.S. Pat. No. 4,202,990, various alcohols are treated with (meth)acrylic esters in the presence of a zirconium acetylacetonate catalyst to produce transester products in high yields. Also, U.S. Pat. No. 4,609,755 describes the activation of Ti/Zr alkoxides by Mg, Ca, and Ba alkoxides for ester interchange of (meth)acrylic esters. Some of the deficiencies mentioned above still exist, however. I have now found that it is possible to achieve very high yield of transesterified ester products by reaction of a lower alkyl ester with an appropriate alcohol in the presence of a hafnium(IV) 1,3-dicarbonyl complex, which complex shows unexpectedly high activity without the subsequent precipitation or insolubility problems associated with the prior art catalysts.